EP1937757A1 - Method for the fabrication of high surface area ratio and high aspect ratio surfaces on substrates - Google Patents

Method for the fabrication of high surface area ratio and high aspect ratio surfaces on substrates

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Publication number
EP1937757A1
EP1937757A1 EP06710193A EP06710193A EP1937757A1 EP 1937757 A1 EP1937757 A1 EP 1937757A1 EP 06710193 A EP06710193 A EP 06710193A EP 06710193 A EP06710193 A EP 06710193A EP 1937757 A1 EP1937757 A1 EP 1937757A1
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EP
European Patent Office
Prior art keywords
plasma
polymer
fabrication
etching
surface area
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EP06710193A
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German (de)
French (fr)
Other versions
EP1937757B1 (en
Inventor
Angeliki Tserepi
Evangelos Gogolides
Konstantinos Misiakos
Maria-Elena Vlachopoulou
Nikolaos Vourdas
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Gogolidis Evangelos
National Center for Scientific Research Demokritos
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Gogolidis Evangelos
National Center for Scientific Research Demokritos
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Publication of EP1937757A1 publication Critical patent/EP1937757A1/en
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Classifications

    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B05—SPRAYING OR ATOMISING IN GENERAL; APPLYING FLUENT MATERIALS TO SURFACES, IN GENERAL
    • B05D—PROCESSES FOR APPLYING FLUENT MATERIALS TO SURFACES, IN GENERAL
    • B05D5/00—Processes for applying liquids or other fluent materials to surfaces to obtain special surface effects, finishes or structures
    • B05D5/08—Processes for applying liquids or other fluent materials to surfaces to obtain special surface effects, finishes or structures to obtain an anti-friction or anti-adhesive surface
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B05—SPRAYING OR ATOMISING IN GENERAL; APPLYING FLUENT MATERIALS TO SURFACES, IN GENERAL
    • B05D—PROCESSES FOR APPLYING FLUENT MATERIALS TO SURFACES, IN GENERAL
    • B05D3/00—Pretreatment of surfaces to which liquids or other fluent materials are to be applied; After-treatment of applied coatings, e.g. intermediate treating of an applied coating preparatory to subsequent applications of liquids or other fluent materials
    • B05D3/14—Pretreatment of surfaces to which liquids or other fluent materials are to be applied; After-treatment of applied coatings, e.g. intermediate treating of an applied coating preparatory to subsequent applications of liquids or other fluent materials by electrical means
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B05—SPRAYING OR ATOMISING IN GENERAL; APPLYING FLUENT MATERIALS TO SURFACES, IN GENERAL
    • B05D—PROCESSES FOR APPLYING FLUENT MATERIALS TO SURFACES, IN GENERAL
    • B05D3/00—Pretreatment of surfaces to which liquids or other fluent materials are to be applied; After-treatment of applied coatings, e.g. intermediate treating of an applied coating preparatory to subsequent applications of liquids or other fluent materials
    • B05D3/14—Pretreatment of surfaces to which liquids or other fluent materials are to be applied; After-treatment of applied coatings, e.g. intermediate treating of an applied coating preparatory to subsequent applications of liquids or other fluent materials by electrical means
    • B05D3/141—Plasma treatment
    • B05D3/142—Pretreatment
    • B05D3/144—Pretreatment of polymeric substrates
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B29—WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
    • B29C—SHAPING OR JOINING OF PLASTICS; SHAPING OF MATERIAL IN A PLASTIC STATE, NOT OTHERWISE PROVIDED FOR; AFTER-TREATMENT OF THE SHAPED PRODUCTS, e.g. REPAIRING
    • B29C59/00—Surface shaping of articles, e.g. embossing; Apparatus therefor
    • B29C59/14—Surface shaping of articles, e.g. embossing; Apparatus therefor by plasma treatment
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B81—MICROSTRUCTURAL TECHNOLOGY
    • B81B—MICROSTRUCTURAL DEVICES OR SYSTEMS, e.g. MICROMECHANICAL DEVICES
    • B81B1/00—Devices without movable or flexible elements, e.g. microcapillary devices
    • C—CHEMISTRY; METALLURGY
    • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
    • C08J—WORKING-UP; GENERAL PROCESSES OF COMPOUNDING; AFTER-TREATMENT NOT COVERED BY SUBCLASSES C08B, C08C, C08F, C08G or C08H
    • C08J7/00—Chemical treatment or coating of shaped articles made of macromolecular substances
    • C08J7/12—Chemical modification
    • H—ELECTRICITY
    • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
    • H05H—PLASMA TECHNIQUE; PRODUCTION OF ACCELERATED ELECTRICALLY-CHARGED PARTICLES OR OF NEUTRONS; PRODUCTION OR ACCELERATION OF NEUTRAL MOLECULAR OR ATOMIC BEAMS
    • H05H1/00—Generating plasma; Handling plasma
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01L—CHEMICAL OR PHYSICAL LABORATORY APPARATUS FOR GENERAL USE
    • B01L3/00—Containers or dishes for laboratory use, e.g. laboratory glassware; Droppers
    • B01L3/50—Containers for the purpose of retaining a material to be analysed, e.g. test tubes
    • B01L3/502—Containers for the purpose of retaining a material to be analysed, e.g. test tubes with fluid transport, e.g. in multi-compartment structures
    • B01L3/5027—Containers for the purpose of retaining a material to be analysed, e.g. test tubes with fluid transport, e.g. in multi-compartment structures by integrated microfluidic structures, i.e. dimensions of channels and chambers are such that surface tension forces are important, e.g. lab-on-a-chip
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B05—SPRAYING OR ATOMISING IN GENERAL; APPLYING FLUENT MATERIALS TO SURFACES, IN GENERAL
    • B05D—PROCESSES FOR APPLYING FLUENT MATERIALS TO SURFACES, IN GENERAL
    • B05D7/00—Processes, other than flocking, specially adapted for applying liquids or other fluent materials to particular surfaces or for applying particular liquids or other fluent materials
    • B05D7/02—Processes, other than flocking, specially adapted for applying liquids or other fluent materials to particular surfaces or for applying particular liquids or other fluent materials to macromolecular substances, e.g. rubber
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B29—WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
    • B29K—INDEXING SCHEME ASSOCIATED WITH SUBCLASSES B29B, B29C OR B29D, RELATING TO MOULDING MATERIALS OR TO MATERIALS FOR MOULDS, REINFORCEMENTS, FILLERS OR PREFORMED PARTS, e.g. INSERTS
    • B29K2033/00—Use of polymers of unsaturated acids or derivatives thereof as moulding material
    • B29K2033/04—Polymers of esters
    • B29K2033/12—Polymers of methacrylic acid esters, e.g. PMMA, i.e. polymethylmethacrylate
    • C—CHEMISTRY; METALLURGY
    • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
    • C08J—WORKING-UP; GENERAL PROCESSES OF COMPOUNDING; AFTER-TREATMENT NOT COVERED BY SUBCLASSES C08B, C08C, C08F, C08G or C08H
    • C08J2333/00—Characterised by the use of homopolymers or copolymers of compounds having one or more unsaturated aliphatic radicals, each having only one carbon-to-carbon double bond, and only one being terminated by only one carboxyl radical, or of salts, anhydrides, esters, amides, imides, or nitriles thereof; Derivatives of such polymers
    • C08J2333/04—Characterised by the use of homopolymers or copolymers of compounds having one or more unsaturated aliphatic radicals, each having only one carbon-to-carbon double bond, and only one being terminated by only one carboxyl radical, or of salts, anhydrides, esters, amides, imides, or nitriles thereof; Derivatives of such polymers esters
    • C08J2333/06—Characterised by the use of homopolymers or copolymers of compounds having one or more unsaturated aliphatic radicals, each having only one carbon-to-carbon double bond, and only one being terminated by only one carboxyl radical, or of salts, anhydrides, esters, amides, imides, or nitriles thereof; Derivatives of such polymers esters of esters containing only carbon, hydrogen, and oxygen, the oxygen atom being present only as part of the carboxyl radical
    • C08J2333/10—Homopolymers or copolymers of methacrylic acid esters
    • C08J2333/12—Homopolymers or copolymers of methyl methacrylate
    • C—CHEMISTRY; METALLURGY
    • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
    • C08J—WORKING-UP; GENERAL PROCESSES OF COMPOUNDING; AFTER-TREATMENT NOT COVERED BY SUBCLASSES C08B, C08C, C08F, C08G or C08H
    • C08J2383/00—Characterised by the use of macromolecular compounds obtained by reactions forming in the main chain of the macromolecule a linkage containing silicon with or without sulfur, nitrogen, oxygen, or carbon only; Derivatives of such polymers
    • C08J2383/04—Polysiloxanes

Definitions

  • This invention relates to a method of fabricating randomly rough columnar-like surfaces of high surface area ratio on polymer/plastic materials and to the application of such for the control of the wetting properties of such surfaces, of the liquid transport on surfaces made employing this method, and of the separation of liquids moving in microchannels the interior surfaces of which are modified according to the method described in this invention.
  • the wettability is an important property of a surface in practical applications, governed by both the chemical nature (surface free energy) and the geometrical structure (surface roughness) of the surface.
  • Two descriptions have been proposed in the art to describe the dependence of the wetting behavior of a surface on the surface roughness: the Wenzel and the Cassie-Baxter description.
  • the Wenzel model (R.N. Wenzel, Ind. Eng. Chem. 1936, 28, p.
  • a Wenzel surface is referred to as "sticky” and is characterized by a high contact angle hysteresis and a high tilt angle
  • a Cassie-Baxter surface is referred to as "slippery” characterized by a small contact angle hysteresis and a small tilt angle
  • the Cassie-Baxter state of superhydrophobic surfaces is characterized as "slippery", in the sense that a droplet does not stick but rolls on such surface.
  • the pressure-driven flow of liquids inside microchannels with super-hydrophobic interior surfaces necessitates smaller pressure difference to maintain the flow.
  • Surfaces of high aspect ratio have recently attracted interest for use in microfluidic devices for analytical purposes, where the chemistry of the walls becomes a significant component in the reaction.
  • microstructures of high aspect ratio have been used in microfluidic devices for liquid chromatography or capillary electrochromatography, for providing high surface to volume ratio leading to enhancement of the analytical performance of the device (N. Lion et al., Electrophoresis 2003, 24, p.
  • TiO 2 nanoparticles mixed with fluorinated copolymers were coated on substrates by spraying (C-T. Hsieh et al., Appl. Surf. Sci. 2005, 240, p. 318) to yield fluorinated surfaces of high surface roughness.
  • Electrochemical deposition of Gold (F. Shi, et al., Adv. Mater. 2005, 17(8), p. 1005) and Silver (N. Zhao, et al., Langmuir 2005, 21, p. 4713) aggregates followed by chemisorption of a monolayer of n-dodecanothiol has been used for creating nanostructured surfaces of super-hydrophobic character.
  • Electro-deposition of Copper has been also used in combination with lithography and etching to result in rough and patterned surfaces (dual scale roughness) of high surface area, which when hydrophobized gave superhydrophobic surfaces (NJ. Shirtcliffe, et al., Adv. Mater. 2004, 16(21), p. 1929, and Langmuir 2005, 21, p. 937).
  • High-aspect-ratio-structured surfaces have been obtained by either polymer nanof ⁇ bers and aligned carbon nanotubes (Lin Feng, et al., Adv. Mat. 2002, 14(24), p.1857) or by photolithography and plasma etching of Si (M.
  • a common feature of all the above mentioned methodologies is that the size and density of the (random or ordered) microstructures on the surfaces can be changed controllably in a way that water contact angles can increase with roughness and a transition between Wenzel type and Cassie-Baxter type (super-hydrophobic) surfaces is observed.
  • the Cassie-Baxter surface state is not stable, as the corresponding energy is a local but not the global minimum (N. Patankar, Langmuir 2003, 19, p.
  • the present invention provides a simple and fast process for the fabrication of random columnar-like high aspect ratio surfaces on a commercial silicone, a widespread material for the fabrication of microfluidic devices or any organic-inorganic polymer. Contrary to the above mentioned state of the art (N. Patankar, 2003) which requires that nanolithography is a necessary step for the fabrication of nanostructures, of which high aspect ratio nanolithography has high equipment and execution costs, the method provided by the present invention overcomes this step and thus, may be fulfilled in any laboratory having plasma etching equipment.
  • the process described in the present invention may be applied for the fabrication of (a) surfaces of controlled wetting properties, (b) surfaces exhibiting low friction against droplet motion and the use of electric fields for control of droplet motion on these surfaces, (c) interior surfaces of micro-channels requiring reduced pressure for pressure- driven flow of liquids through these channels, (d) interior surfaces of micro-channels used in liquid chromatography or capillary electrochromatography or capillary electrophoresis of analytical microdevices. (e) surfaces of plastics with controlled wetting properties in combination with the desired transparency.
  • the method of the present invention is applied, among others, to the plastics industry, micro-analytical devices, so called “smart” and self-cleaning surfaces of any size and nature, such as for example for the formation of substrates on glasswalls (self- cleaning glasses), on anti-corrosion surfaces, on vehicles, buildings etc. for protection of any surface exposed to the air and on which it is desirable to avoid deposition of pollutant particles.
  • the surfaces on which the columnar structures are made are polymer/ plastic.
  • the polymer contains elements in its volume and/or on its surface which have different etching behaviour (etchable versus un-etchable) in the plasma used.
  • This invention also provides a method for the control of the wetting properties of the above surface, using columnar-like high aspect ratio structures with a proper chemical modification in plasmas, so as to render it highly hydrophilic or super-hydrophobic.
  • Figure 1 is an SEM image of a tilted silicone surface (PDMS) after a 6 min SF 6 plasma treatment (initial film thickness: 6 ⁇ m). Nano-"grass” (i.e. nano-collumns) 1.45 ⁇ m-high is shown.
  • Figure 2 is an AFM top view image of a silicone surface (PDMS) after 2 min treatment in SF 6 plasma. Roughness analysis gave an rms value of 133 nm and a periodicity of 240 nm. Initial film thickness: 20 ⁇ m.
  • Figure 3 is an image of a water droplet- advancing 157°, receding 154°, hysteresis 3°- on a silicone surface (PDMS) after 2 min SF 6 plasma treatment and deposition of a 20 nm-thick fluorocarbon film. Initial PDMS film thickness: 2 ⁇ m.
  • Figure 4 is an image of a water droplet during its rolling off (after being thrown on the surface) on a 6 ⁇ m thick silicone (PDMS) layer after treated for 6 min in SF 6 plasma and deposition of a 20 nm-thick fluorocarbon film.
  • PDMS silicone
  • Figure 5 shows line scans using a stylus profilometer of the surface of Plexiglass (PMMA- poly(methylmethacrylate)) sheets after Oxygen plasma treatment at various times (10, 30, 60, 120 min), as well as Atomic Force Microscope (AFM) scans after Oxygen plasma treatment for 2 and 5min.
  • AFM Atomic Force Microscope
  • Figure 6 is an image of a water droplet - advancing 155°, receding 148° , hysteresis 7°- on a commercial Plexiglas (PMMA-poly(methylmethacrylate) surface after 30min Oxygen plasma treatment in a high density plasma reactor, and spin coating deposition of a 20nm fluorocarbon film.
  • the droplet is actually rolling on the plexiglass surface.
  • Figure 7 shows the variation of the static, and advancing contact angle and hysteresis of a Plexiglass surface after Oxygen plasma treatment and spin-coating with 20nm fluorocarbon polymer, as a function of the time of plasma treatment.
  • the surface becomes superhydrophobic after only 15min of plasma treatment. Shorter times of treatment are possible when plasma deposited fluorocarbon polymer is used.
  • FIG. 8 shows Three-Dimensional Atomic-Force-Microscope (AFM) images of Plexiglass surfaces after Oxygen plasma treatment for 2 min (Fig. 8a) and 5 min (Fig. 8b).
  • AFM Atomic-Force-Microscope
  • the surface area ratio is 1.7 for 2min and 2.4 for 5min oxygen plasma treatment.
  • a plasma treatment only between 5 to lOmin is enough to create high surface area ratio and lead to superhydrophobic surfaces after coating with plasma deposited fluorocarbon layer, which conforms better to the topography compared to the spin-coated fluorocarbon layer.
  • the present invention presents a novel and easy to implement method for fabricating surfaces of high aspect ratio, based on plasma treatment of poly-dimethyl siloxane (PDMS).
  • PDMS contains organic methyl (-CH 3 ) groups attached on an inorganic backbone (-[Si-O] n -).
  • the present invention presents a novel and easy to implement technique for fabricating surfaces of high aspect ratio, based on plasma treatment of any other polymeric material containing elements of different plasma etching behavior.
  • any commercial organic polymer/plastic such as Plexiglass (PMMA- poly(methylmethacrylate)) which contains components with different plasma etching behavior, and/or small amounts ( ⁇ 1%) of metallic elements (such as Al, Fe) as contaminants from the fabrication process of the polymer/plastic may be used for the realization of the invention.
  • PMMA- poly(methylmethacrylate) which contains components with different plasma etching behavior, and/or small amounts ( ⁇ 1%) of metallic elements (such as Al, Fe) as contaminants from the fabrication process of the polymer/plastic may be used for the realization of the invention.
  • any commercial organic polymer /plastic such as Plexiglass (PMMA- poly(methylmethacrylate)) which may have in addition to volume contaminants also surface contaminants (such as metallic or semiconducting elements or other material with different etching behaviour compared to that of the polymer) due to a previous sputtering or other nanometer thick coating process, may be used for the realization of the invention.
  • PMMA- poly(methylmethacrylate) which may have in addition to volume contaminants also surface contaminants (such as metallic or semiconducting elements or other material with different etching behaviour compared to that of the polymer) due to a previous sputtering or other nanometer thick coating process.
  • organic groups are expected to etch much faster compared to their inorganic etch resistant counterparts, which contribute to the formation of non- volatile products on etched surfaces and thus behave as masking materials to etching.
  • siloxane As basic material, a commercial siloxane (Silguard 184 supplied by Dow Corning), thermally curable, is used. This is an innate hydrophobic material, widely used in the fabrication of microfluidic devices, giving a water contact angle of 105°.
  • siloxane films by spin coating of the polymer and curing agent mixture on a substrate surface and subsequent thermal curing to result in 2 ⁇ m-2mm -thick films. These films are then exposed to SF 6 plasma treatment, in an inductively coupled plasma (ICP) reactor, under conditions ensuring high etch rates (pressure of 10 mTorr, plasma power 1900 W, and bias voltage -100 V).
  • ICP inductively coupled plasma
  • Figure 1 shows an SEM image of a PDMS surface after a 6 min treatment in SF 6 plasma. This image reveals densely packed nanocollumns on the surface of PDMS of average height of 1.45 ⁇ m and of diameter of the order of 100 nm. This dense distribution of nanocollumns on the PDMS surface is further verified by AFM imaging, as shown in Fig. 2. An average collumn diameter of 130 nm is indicated and a surface fraction of about 25% can be calculated as covered by the collumn tops.
  • FC fluorocarbon film
  • the enhancement of the surface hydrophobicity from 118° (contact angle on a flat FC surface) to 147° (contact angle on a nano-textured surface) as given by the Cassie-Baxter Equation (2) indicates a water droplet in contact with the nano-collumn tops covering a surface fraction ⁇ s equal to 30%, in good agreement with the estimation of the surface fraction covered by the collumn top surface based on the AFM image in Fig. 2.
  • siloxane surfaces treated as described above are used as surfaces of minimal friction to water droplet motion. This is due to the small contact area of the droplets with the surface and the low contact angle hysteresis, as it was described in the previous paragraph. Therefore, the motion of droplets on such surfaces can be controlled by external forces such as the electrostatic force exerted on a droplet resting on a siloxane plasma-treated surface by means of a voltage applied on an electrode located to a small distance from the droplet.
  • relatively small voltages (below 100 V) applied by a metallic pin located a few hundred microns away from the droplet are sufficient to cause droplet motion.
  • Example 3 In another preferred embodiment of the present invention, the method described above for fabrication of superhydrophobic surfaces is used to modify the interior surface of microchannels fabricated in PDMS or in other polymeric material through soft lithography or other patterning technique (for example, photolithography). If the flow inside the microchannels is pressure-driven, a small pressure difference would suffice to maintain the flow inside the microchannels, due to the non-sticky but slippery nature of the superhydrophobic interior surfaces of the microchannels.
  • the method described above is used for the fabrication of columnar-like nanostructures inside the microchannels of an analytical microdevice (such as those used for liquid chromatography, or capillary electrochromatography or capillary electrophoresis or other relevant analytical technique).
  • an analytical microdevice such as those used for liquid chromatography, or capillary electrochromatography or capillary electrophoresis or other relevant analytical technique.
  • the height of the nano-columns is comparable to the height of the microchannel, and the density of the fabricated nanostructures can be adjusted according to the surface-to-volume requirements of the device for enhanced analytical performance (high resolution, short analysis time).
  • the high surface area of the very rough microchannels can be used for adjustment of the zeta potential (surface charge) and the electroosmotic flow.
  • Example 5 Example 5:
  • Plexiglass Poly(methylmethacrylate) are treated in an oxygen plasma.
  • the commercially available polymer in thicknesses from 1 to several mm, includes less than 1% metallic impurities which do not etch in the oxygen plasma, as we verified by EDAX analysis of the sheets.
  • Plexiglass has a smooth surface, is transparent to visible light, and has a contact angle of 60°, and is a material widely used in microfluidic devices.
  • ICP inductively coupled plasma
  • FIG. 5 shows line scans using a profilometer or an Atomic Force Microscope (AFM) of a plexiglass surface after treatment in Oxygen plasma for several instances. This image reveals nanocolumns on the surface of plexiglass of peak to peak height ranging from 0.25 to 8 ⁇ m depending on the plasma treatment time.
  • AFM Atomic Force Microscope
  • FC hydrophobicity results in the enhancement of the contact angle of water droplets on the surface to more than 150° (Fig. 6), after a 15min treatment in Oxygen plasma.
  • the dense "forest" of nano- collumns shown in the profilometer or AFM scans (Fig. 5) as well as the AFM images of Figures 8a and 8b cannot allow the water to penetrate and wet the surface of the collumns, thus water droplets are sitting on a composite solid and air surface, leading to low values of contact angle hysteresis (average of 7°).
  • the low hysteresis is further evidenced by the rolling of water droplets when thrown on the surface (Fig. 6) and the inability to detach a droplet from the needle using contact with the surface. All these observations provide evidence that water droplets on these surfaces behave according to the Cassie-Baxter model.
  • treatment times refer to spin coated fluorocarbon films, while shorter times are obtained with plasma deposited fluorocarbon films, when the surface becomes syperhydrophobic after approximately 5-10min of oxygen plasma treatment and plasma fluorocarbon deposition.
  • AFM images of Figures 8a and 8b show that the surface area ratio is 1.7 for 2min treatment, while it raises to 2.4 for 5min treatment.
  • the superhydrophobic plexiglass or other oxygen plasma treated organic polymer may however be milky (less transparent) after the plasma treatment.
  • the surface roughness and wettability can be adjusted without sacrificing the transparency of the plastic and in fact tuning the minimum cutoff wavelength of light transmission through the plastic.
  • This can be accomplished by increasing the amount of unetchable metallic or other elements that exist as contaminants in the plastic and or changing the plasma etching conditions, such as the bias voltage.
  • the surface roughness increased after oxygen plasma treatment and the surfaces become superhydrophobic after Teflon coating before losing their transparency.
  • Plexiglass surfaces treated as described above are used as surfaces of minimal friction to water droplet motion. This is due to the small contact area of the droplets with the surface and the low contact angle hysteresis, as it was described in the previous paragraph.
  • the method described above for the fabrication of superhydrophobic surfaces is used to modify the interior surface of microchannels fabricated in Plexiglass. If the flow inside the microchannels is pressure- driven, a small pressure difference would suffice to maintain the flow inside the microchannels, due to the non-sticky but slippery nature of the superhydrophobic interior surfaces of the microchannels.
  • Example 9
  • the method described above was used for the fabrication of columnar-like nanostructures inside the microchannels of an analytical microdevice (such as those used for liquid chromatography, or capillary electrochromatography or capillary electrophoresis or other relevant analytical technique).
  • the height of the nano-columns may be comparable to the height of the microchannel, and the density of the fabricated nanostructures can be adjusted according to the surface-to- volume requirements of the device for enhanced analytical performance (high resolution, short analysis time).
  • the high surface area of the very rough microchannels can be used for adjustment of the zeta potential (surface charge) and the electroosmotic flow.

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Abstract

This invention provides a method for the fabrication of surfaces of high surface area ratio on polymeric/plastic materials, and their application in the control of the wetting properties of surfaces, of the transport of liquids on such fabricated surfaces, or of the separation of liquids in microchannels of said surfaces. The fabrication of surfaces of high surface area ratio comprises the following steps : (a) selection of a polymer/plastic layer which contains two or more componenets differing in their plasma etching behaviour (b) exposure of said polymer/plastic layer to an etching plasma to provide selective removal of one polymer component versus a second plasma-resistant component so as to result in a randomly rough columnar-like surface. In addition, exposure of the said surface to an oxidizing plasma or to a fluorocarbon. depositing plasma renders the surface fully hydrophilic or super-hydrophobic, respectively.

Description

"METHOD FOR THE FABRICATION OF HIGH SURFACE AREA RATIO AND HIGH ASPECT RATIO SURFACES ON SUBSTRATES"
DESCRIPTION OF THE INVENTION
Field of the Invention
This invention relates to a method of fabricating randomly rough columnar-like surfaces of high surface area ratio on polymer/plastic materials and to the application of such for the control of the wetting properties of such surfaces, of the liquid transport on surfaces made employing this method, and of the separation of liquids moving in microchannels the interior surfaces of which are modified according to the method described in this invention. Description of the related state of the art
The wettability is an important property of a surface in practical applications, governed by both the chemical nature (surface free energy) and the geometrical structure (surface roughness) of the surface. Two descriptions have been proposed in the art to describe the dependence of the wetting behavior of a surface on the surface roughness: the Wenzel and the Cassie-Baxter description. The Wenzel model (R.N. Wenzel, Ind. Eng. Chem. 1936, 28, p. 988) holds for cases where the liquid remains in contact with the solid surface below the liquid droplet and the enhancement in hydrophilicity or hydrophobicity by the surface roughness is quantitatively given by the following equation for the observed contact angle θ'w: cosθ'w = rcosθ (1) where θ is the equilibrium contact angle on a flat surface of the same chemical character, and r is the ratio of the actual surface area to the projected area of the rough surface. The Cassie-Baxter model holds for cases where the surface topography is such that water cannot fully penetrate (for example in thin deep channels on the surface of the material) and air is trapped below the droplet which is suspended across surface protrusions. In such cases, the droplet is in contact with a composite solid and vapor surface, and thus the observed contact angle θ'c is given by the following equation: cosθ'c= Φs cosθ-(l- Φs) (2) where φs is the fraction of the solid surface upon which the droplet sits. Besides the difference between the predictions for the observed contact angle θ'w and θ'c in the two cases, one important difference between the Wenzel and the Cassie-Baxter models is that related to the ease of droplet motion on surfaces: a Wenzel surface is referred to as "sticky" and is characterized by a high contact angle hysteresis and a high tilt angle, while a Cassie-Baxter surface is referred to as "slippery" characterized by a small contact angle hysteresis and a small tilt angle (G. McHaIe et al., Analyst 2004, 129, p.284, and Langmuir 2004, 20, p. 10146). No matter the differences between the above mentioned descriptions, they both call for surfaces of increased surface area ratio for achieving enhancement in surface wetting properties. In the last years, the application of super-hydrophobic surfaces as water-repellent coatings has attracted significant attention for exterior applications, since several phenomena such as snow sticking or water resistance are expected to be reduced on such surfaces. In order to avoid decrease of this property with outdoors use, coating of such surfaces with photocatalysts has been proposed for attaining degradation of pollutants on superhydrophobic surfaces, thus enhancing the self-cleaning ability of such type of surfaces (H. Yamashita, et al., Nucl. Instr. andMeth. in Phys. Res. B 206 (2003), p. 898). Besides water-repellency, another important characteristic of a superhydrophobic surface is its low hysteresis (difference between advancing and receding contact angles), which results in easing of droplet motion on such surfaces by use of low tilt angles or low driving forces (G. McHaIe et al., Analyst 2004, 129, p.284). Such forces can be applied by external fields such as static electric fields, which have been used to control the droplet motion on superhydrophobic surfaces (K. Takeda et al., Jpn. J. Appl Phys. 41, 2002, p. 287). In addition, the Cassie-Baxter state of superhydrophobic surfaces is characterized as "slippery", in the sense that a droplet does not stick but rolls on such surface. Thus, the pressure-driven flow of liquids inside microchannels with super-hydrophobic interior surfaces necessitates smaller pressure difference to maintain the flow. Surfaces of high aspect ratio have recently attracted interest for use in microfluidic devices for analytical purposes, where the chemistry of the walls becomes a significant component in the reaction. For example, microstructures of high aspect ratio have been used in microfluidic devices for liquid chromatography or capillary electrochromatography, for providing high surface to volume ratio leading to enhancement of the analytical performance of the device (N. Lion et al., Electrophoresis 2003, 24, p. 3533). For the fabrication of surfaces of high surface area ratio (the actual surface area divided by the projected area), a lot of effort, inspired in many cases from the natural world (Lin Feng, et al., Adv. Mat. 2002, 14(24), p.1857), has focused on the development of different methodologies for designing micro- or nano-textured surfaces on a variety of materials. Polymeric films (polyurethane, polyvinylidene fluoride) were templated on materials such as porous alumina or silica colloidal assemply, respectively, (X. Zhao and W. Li, Surface & Coatings Technology 2004, and J. Li et al., Appl. Surf. Sci. 2005) in order to modulate their morphology and obtain enhanced surface roughness. TiO2 nanoparticles mixed with fluorinated copolymers were coated on substrates by spraying (C-T. Hsieh et al., Appl. Surf. Sci. 2005, 240, p. 318) to yield fluorinated surfaces of high surface roughness.
Electrochemical deposition of Gold (F. Shi, et al., Adv. Mater. 2005, 17(8), p. 1005) and Silver (N. Zhao, et al., Langmuir 2005, 21, p. 4713) aggregates followed by chemisorption of a monolayer of n-dodecanothiol has been used for creating nanostructured surfaces of super-hydrophobic character.
Electro-deposition of Copper has been also used in combination with lithography and etching to result in rough and patterned surfaces (dual scale roughness) of high surface area, which when hydrophobized gave superhydrophobic surfaces (NJ. Shirtcliffe, et al., Adv. Mater. 2004, 16(21), p. 1929, and Langmuir 2005, 21, p. 937). High-aspect-ratio-structured surfaces have been obtained by either polymer nanofϊbers and aligned carbon nanotubes (Lin Feng, et al., Adv. Mat. 2002, 14(24), p.1857) or by photolithography and plasma etching of Si (M. Callies, et al., Microelectronic Engineering 2005, 78-79, p. 100), to obtain superhydrophobic surfaces. Plasma techniques have been also used to either modify the chemistry and the surface morphology of polymeric surfaces such as a) polybutadiene to render them superhydrophobic through fluorination and surface roughening (I. Woodward, et al., Langmuir 2003, 19, p. 3432), b) Polypropelene to render the surface superhydrophobic through roughening by etching faster the crystalline versus the amorphous phase and co-sputtering of Poly(tetrafluoroethylene) (J. P. Youngblood and T. J. McCarthy Macromolecules 1999, 32, p. 6800), c) Poly(ethylene terephthalate) to render the surface superhydrophobic by etching faster the linear versus the phenolic parts of the polymer chain using mild plasma conditions and subsequent deposition of hydrophobic polymer (K. Teshima et al. Appl Surf. Set 2005, 244 p. 619), or d) to deposit ribbon-like randomly-distributed microstructures used as super-hydrophobic fluorocarbon coatings (P. Favia, et al., Surface & Coatings Technology 2003, 169-170, ρ.609).
In addition, plasma over-etching of a photoresist layer coated on Si followed by fluorocarbon plasma deposition has been used for the fabrication of surfaces of tunable hydrophobicity depending on the density of the residual photoresist particles on the Si surface (L.-M. Lacroix et al., Surface Science 2005).
However, methods a,b,c, described above, which use some selective plasma nanotexturing, present the problem that their application is restricted to the properties of specific polymers, and that they use mild conditions, which are not suitable for fast and anisotropic etching, which is desired for the fabrication of microfluidic devices. Further, although methods a, b, c, d described above as well as the method of L.-M. Lacroix et al., Surface Science 2005, fabricate surfaces of high surface area, they do not achieve surfaces of high aspect ratio, which would obtain robust hydrophobic substrates.
Therefore, what is missing from the state of the art is a generic fast method suitable for different polymers/plastics (such as purely organic or also inorganic containing) and suitable for different plasma gas chemistries, which permit anisotropic etching.
A common feature of all the above mentioned methodologies is that the size and density of the (random or ordered) microstructures on the surfaces can be changed controllably in a way that water contact angles can increase with roughness and a transition between Wenzel type and Cassie-Baxter type (super-hydrophobic) surfaces is observed. Usually, the Cassie-Baxter surface state is not stable, as the corresponding energy is a local but not the global minimum (N. Patankar, Langmuir 2003, 19, p. 1249), and the transition from composite (Cassie-Baxter) to wetted (Wenzel) contact is followed by a reduction in the contact angle (θ'c > θ'w)- To obtain a robust substrate characterized by a minimal difference between θ'c and θ'w, we need to follow specific design rules (N. Patankar, Langmuir 2003, 19, p. 1249) which dictate the use of tall slender columns as microstructures on surfaces. Such requirements (nano-collumns and contact angles) cannot be easily satisfied by the known until now standard patterning techniques (lithography and plasma etching).
The present invention provides a simple and fast process for the fabrication of random columnar-like high aspect ratio surfaces on a commercial silicone, a widespread material for the fabrication of microfluidic devices or any organic-inorganic polymer. Contrary to the above mentioned state of the art (N. Patankar, 2003) which requires that nanolithography is a necessary step for the fabrication of nanostructures, of which high aspect ratio nanolithography has high equipment and execution costs, the method provided by the present invention overcomes this step and thus, may be fulfilled in any laboratory having plasma etching equipment.
In addition it provides a simple process for the fabrication of random columnar-like high aspect ratio surfaces on a commercial Plexiglas (PMMA-poly(methylmethacrylate)) or any commercial organic polymer containing components with different plasma etching behavior, un-etchable impurities or additives, as well as on any other material containing elements with different plasma etching behavior. The process described in the present invention may be applied for the fabrication of (a) surfaces of controlled wetting properties, (b) surfaces exhibiting low friction against droplet motion and the use of electric fields for control of droplet motion on these surfaces, (c) interior surfaces of micro-channels requiring reduced pressure for pressure- driven flow of liquids through these channels, (d) interior surfaces of micro-channels used in liquid chromatography or capillary electrochromatography or capillary electrophoresis of analytical microdevices. (e) surfaces of plastics with controlled wetting properties in combination with the desired transparency.
In practice, the method of the present invention is applied, among others, to the plastics industry, micro-analytical devices, so called "smart" and self-cleaning surfaces of any size and nature, such as for example for the formation of substrates on glasswalls (self- cleaning glasses), on anti-corrosion surfaces, on vehicles, buildings etc. for protection of any surface exposed to the air and on which it is desirable to avoid deposition of pollutant particles.
Summary of the invention
It is an object of the present invention to provide a fast method of fabricating columnar- like high aspect ratio structures on surfaces by means of plasma etching techniques.
Preferably, the surfaces on which the columnar structures are made are polymer/ plastic.
The polymer contains elements in its volume and/or on its surface which have different etching behaviour (etchable versus un-etchable) in the plasma used.
This invention also provides a method for the control of the wetting properties of the above surface, using columnar-like high aspect ratio structures with a proper chemical modification in plasmas, so as to render it highly hydrophilic or super-hydrophobic.
It is yet another object of this invention to provide a simultaneous control of the wetting properties plus transparency of polymeric/plastic substrates.
It is yet another object of this invention to provide a method for controlling the transport of liquid droplets on such superhydrophobic surfaces (characterized by low friction to droplet motion) by means of electric fields or of liquids inside microchannels the interior surface of which is made super-hydrophobic according to the process described in this invention.
It is yet another object of this invention to use the highly hydrophilic surfaces fabricated according to the present invention as the interior surfaces of a micro-channel of a liquid chromatography or capillary electrochromatography or capillary electrophoresis microdevice, where said structures are used as microfabricated support structures.
Brief description of drawings The invention is illustrated in the following figures:
Figure 1 is an SEM image of a tilted silicone surface (PDMS) after a 6 min SF6 plasma treatment (initial film thickness: 6 μm). Nano-"grass" (i.e. nano-collumns) 1.45 μm-high is shown.
Figure 2 is an AFM top view image of a silicone surface (PDMS) after 2 min treatment in SF6 plasma. Roughness analysis gave an rms value of 133 nm and a periodicity of 240 nm. Initial film thickness: 20μm. Figure 3 is an image of a water droplet- advancing 157°, receding 154°, hysteresis 3°- on a silicone surface (PDMS) after 2 min SF6 plasma treatment and deposition of a 20 nm-thick fluorocarbon film. Initial PDMS film thickness: 2 μm.
Figure 4 is an image of a water droplet during its rolling off (after being thrown on the surface) on a 6 μm thick silicone (PDMS) layer after treated for 6 min in SF6 plasma and deposition of a 20 nm-thick fluorocarbon film.
Figure 5 shows line scans using a stylus profilometer of the surface of Plexiglass (PMMA- poly(methylmethacrylate)) sheets after Oxygen plasma treatment at various times (10, 30, 60, 120 min), as well as Atomic Force Microscope (AFM) scans after Oxygen plasma treatment for 2 and 5min. The columnar structure can be observed
Figure 6 is an image of a water droplet - advancing 155°, receding 148° , hysteresis 7°- on a commercial Plexiglas (PMMA-poly(methylmethacrylate) surface after 30min Oxygen plasma treatment in a high density plasma reactor, and spin coating deposition of a 20nm fluorocarbon film. The droplet is actually rolling on the plexiglass surface.
Figure 7 shows the variation of the static, and advancing contact angle and hysteresis of a Plexiglass surface after Oxygen plasma treatment and spin-coating with 20nm fluorocarbon polymer, as a function of the time of plasma treatment. The surface becomes superhydrophobic after only 15min of plasma treatment. Shorter times of treatment are possible when plasma deposited fluorocarbon polymer is used.
Figure 8 shows Three-Dimensional Atomic-Force-Microscope (AFM) images of Plexiglass surfaces after Oxygen plasma treatment for 2 min (Fig. 8a) and 5 min (Fig. 8b).
The surface area ratio is 1.7 for 2min and 2.4 for 5min oxygen plasma treatment.
Therefore a plasma treatment only between 5 to lOmin is enough to create high surface area ratio and lead to superhydrophobic surfaces after coating with plasma deposited fluorocarbon layer, which conforms better to the topography compared to the spin-coated fluorocarbon layer.
Description of the preferred embodiments
According to a preferred embodiment, the present invention presents a novel and easy to implement method for fabricating surfaces of high aspect ratio, based on plasma treatment of poly-dimethyl siloxane (PDMS). PDMS contains organic methyl (-CH3) groups attached on an inorganic backbone (-[Si-O]n-).
Advantageously, the present invention presents a novel and easy to implement technique for fabricating surfaces of high aspect ratio, based on plasma treatment of any other polymeric material containing elements of different plasma etching behavior.
Advantageously, any commercial organic polymer/plastic such as Plexiglass (PMMA- poly(methylmethacrylate)) which contains components with different plasma etching behavior, and/or small amounts (<1%) of metallic elements (such as Al, Fe) as contaminants from the fabrication process of the polymer/plastic may be used for the realization of the invention.
Advantageously, any commercial organic polymer /plastic such as Plexiglass (PMMA- poly(methylmethacrylate)) which may have in addition to volume contaminants also surface contaminants (such as metallic or semiconducting elements or other material with different etching behaviour compared to that of the polymer) due to a previous sputtering or other nanometer thick coating process, may be used for the realization of the invention. As a result, organic groups are expected to etch much faster compared to their inorganic etch resistant counterparts, which contribute to the formation of non- volatile products on etched surfaces and thus behave as masking materials to etching.
Preferred embodiments of the invention are presented in the following examples:
Example 1 :
As basic material, a commercial siloxane (Silguard 184 supplied by Dow Corning), thermally curable, is used. This is an innate hydrophobic material, widely used in the fabrication of microfluidic devices, giving a water contact angle of 105°. We prepare siloxane films by spin coating of the polymer and curing agent mixture on a substrate surface and subsequent thermal curing to result in 2μm-2mm -thick films. These films are then exposed to SF6 plasma treatment, in an inductively coupled plasma (ICP) reactor, under conditions ensuring high etch rates (pressure of 10 mTorr, plasma power 1900 W, and bias voltage -100 V). For plasma exposures in the range 2-16 min, we observed formation of columnar-like structures or micro- and nano-"grass" with height ranging from a few 100's nm to 6 μm, depending on the treatment time. Figure 1 shows an SEM image of a PDMS surface after a 6 min treatment in SF6 plasma. This image reveals densely packed nanocollumns on the surface of PDMS of average height of 1.45 μm and of diameter of the order of 100 nm. This dense distribution of nanocollumns on the PDMS surface is further verified by AFM imaging, as shown in Fig. 2. An average collumn diameter of 130 nm is indicated and a surface fraction of about 25% can be calculated as covered by the collumn tops.
After exposure to SF6 plasma, a depositing C4F8 plasma is used in the same reactor (ICP), under plasma conditions ensuring conformal deposition of a thin (20-nm-thick) fluorocarbon film (FC) on the formed columnar structures (Pressure 40 mTorr, plasma power 800 W, bias voltage -100 V). The combination of the FC hydrophobicity with the surface nano-texturing results in the enhancement of the contact angle of water droplets on the surface to more than 150° (Fig. 3), even after a short treatment in SF6 plasma (2 min). The dense "forest" of nano-collumns shown in the AFM image (Fig. 2) cannot allow the water to penetrate and wet the surface of the collumns, thus water droplets are sitting on a composite solid and air surface, leading to low values of contact angle hysteresis (average of 3 ). The low hysteresis is further evidenced by the rolling of water droplets when thrown on the surface (Fig. 4) and the inability to detach a droplet from the needle using contact with the surface. All these observations provide evidence that water droplets on these surfaces behave according to the Cassie-Baxter model. In fact, the enhancement of the surface hydrophobicity from 118° (contact angle on a flat FC surface) to 147° (contact angle on a nano-textured surface) as given by the Cassie-Baxter Equation (2) indicates a water droplet in contact with the nano-collumn tops covering a surface fraction φs equal to 30%, in good agreement with the estimation of the surface fraction covered by the collumn top surface based on the AFM image in Fig. 2.
Example 2:
In another preferred embodiment of the present invention, siloxane surfaces treated as described above (first in SF6 and then in C4F8 plasmas) are used as surfaces of minimal friction to water droplet motion. This is due to the small contact area of the droplets with the surface and the low contact angle hysteresis, as it was described in the previous paragraph. Therefore, the motion of droplets on such surfaces can be controlled by external forces such as the electrostatic force exerted on a droplet resting on a siloxane plasma-treated surface by means of a voltage applied on an electrode located to a small distance from the droplet. Experiments performed by the inventors have shown that relatively small voltages (below 100 V) applied by a metallic pin located a few hundred microns away from the droplet are sufficient to cause droplet motion.
Example 3: In another preferred embodiment of the present invention, the method described above for fabrication of superhydrophobic surfaces is used to modify the interior surface of microchannels fabricated in PDMS or in other polymeric material through soft lithography or other patterning technique (for example, photolithography). If the flow inside the microchannels is pressure-driven, a small pressure difference would suffice to maintain the flow inside the microchannels, due to the non-sticky but slippery nature of the superhydrophobic interior surfaces of the microchannels.
Example 4:
In another preferred embodiment of the present invention, the method described above is used for the fabrication of columnar-like nanostructures inside the microchannels of an analytical microdevice (such as those used for liquid chromatography, or capillary electrochromatography or capillary electrophoresis or other relevant analytical technique). Preferably, the height of the nano-columns is comparable to the height of the microchannel, and the density of the fabricated nanostructures can be adjusted according to the surface-to-volume requirements of the device for enhanced analytical performance (high resolution, short analysis time). In addition the high surface area of the very rough microchannels can be used for adjustment of the zeta potential (surface charge) and the electroosmotic flow. Example 5:
Organic polymer sheets of Plexiglass (PMMA-poly(methylmethacrylate) are treated in an oxygen plasma. The commercially available polymer in thicknesses from 1 to several mm, includes less than 1% metallic impurities which do not etch in the oxygen plasma, as we verified by EDAX analysis of the sheets. Plexiglass has a smooth surface, is transparent to visible light, and has a contact angle of 60°, and is a material widely used in microfluidic devices. These sheets are then exposed to O2 plasma treatment, in an inductively coupled plasma (ICP) reactor, under conditions ensuring high etch rates (pressure of 5 mTorr, plasma power 1900 W, and bias voltage -100 V). For plasma exposures in the range 2-90 min, we observed formation of columnar-like structures or micro- and nano-"grass" with height ranging from a few 100's nm to 8 μm, depending on the treatment time. Figure 5 shows line scans using a profilometer or an Atomic Force Microscope (AFM) of a plexiglass surface after treatment in Oxygen plasma for several instances. This image reveals nanocolumns on the surface of plexiglass of peak to peak height ranging from 0.25 to 8 μm depending on the plasma treatment time. After oxygen plasma treatment a fluorocarbon layer is deposited on the surface of the Plexiglass by spin coating or by a depositing plasma as was described above for PDMS surfaces. The combination of the FC hydrophobicity with the surface nano-texturing results in the enhancement of the contact angle of water droplets on the surface to more than 150° (Fig. 6), after a 15min treatment in Oxygen plasma. The dense "forest" of nano- collumns shown in the profilometer or AFM scans (Fig. 5) as well as the AFM images of Figures 8a and 8b cannot allow the water to penetrate and wet the surface of the collumns, thus water droplets are sitting on a composite solid and air surface, leading to low values of contact angle hysteresis (average of 7°). The low hysteresis is further evidenced by the rolling of water droplets when thrown on the surface (Fig. 6) and the inability to detach a droplet from the needle using contact with the surface. All these observations provide evidence that water droplets on these surfaces behave according to the Cassie-Baxter model.
When studying the variation of the water contact angle and contact angle hysteresis versus the time of oxygen plasma treatment and after spin coating with 20nm Teflon-like polymer (Figure 7) we observe that the surface of plexiglass quickly becomes very hydrophobic, but with large hysteresis. At very small treatment times the surface is hydrophobic and "sticky" (i.e. has a large hysteresis), revealing a surface of the Wenzel type, while at medium treatment times longer than 15min the surface becomes superhydrophobic with minimal hysteresis and is non "sticky", thus following the Cassie- Baxter model. These treatment times refer to spin coated fluorocarbon films, while shorter times are obtained with plasma deposited fluorocarbon films, when the surface becomes syperhydrophobic after approximately 5-10min of oxygen plasma treatment and plasma fluorocarbon deposition. Indeed the AFM images of Figures 8a and 8b show that the surface area ratio is 1.7 for 2min treatment, while it raises to 2.4 for 5min treatment. The superhydrophobic plexiglass or other oxygen plasma treated organic polymer may however be milky (less transparent) after the plasma treatment.
Example 6:
In another preferred embodiment of the present invention the surface roughness and wettability can be adjusted without sacrificing the transparency of the plastic and in fact tuning the minimum cutoff wavelength of light transmission through the plastic. This can be accomplished by increasing the amount of unetchable metallic or other elements that exist as contaminants in the plastic and or changing the plasma etching conditions, such as the bias voltage. In this example we sputtered on the surface of plexiglass platinum (or other metal) monolayers or alternatively we created residues and "dirt" using an inorganic polymer or photoresist. The purpose was to increase the amount of unetchable contaminants on the surface of the plastic. The surface roughness increased after oxygen plasma treatment and the surfaces become superhydrophobic after Teflon coating before losing their transparency.
Example 7:
In another preferred embodiment of the present invention, Plexiglass surfaces treated as described above (first in oxygen and then in C4F8 plasmas) are used as surfaces of minimal friction to water droplet motion. This is due to the small contact area of the droplets with the surface and the low contact angle hysteresis, as it was described in the previous paragraph.
Example 8:
In another preferred embodiment of the present invention, the method described above for the fabrication of superhydrophobic surfaces is used to modify the interior surface of microchannels fabricated in Plexiglass. If the flow inside the microchannels is pressure- driven, a small pressure difference would suffice to maintain the flow inside the microchannels, due to the non-sticky but slippery nature of the superhydrophobic interior surfaces of the microchannels. Example 9:
It is another preferred embodiment of the present invention, the method described above was used for the fabrication of columnar-like nanostructures inside the microchannels of an analytical microdevice (such as those used for liquid chromatography, or capillary electrochromatography or capillary electrophoresis or other relevant analytical technique). The height of the nano-columns may be comparable to the height of the microchannel, and the density of the fabricated nanostructures can be adjusted according to the surface-to- volume requirements of the device for enhanced analytical performance (high resolution, short analysis time). In addition the high surface area of the very rough microchannels can be used for adjustment of the zeta potential (surface charge) and the electroosmotic flow.

Claims

Claims
1. A method for the fabrication of randomly rough columnar-like surfaces of high surface area ratio on substrates, characterized in that it comprises the following steps : (a) selection of a material comprising two or more components which differ with respect to their etching behavior and (b) exposure of said layer to an etching plasma under conditions appropriate to provide selective removal of one component of the material versus a second plasma-resistant component.
2. A method for the fabrication of randomly rough columnar-like surfaces of high surface area ratio on polymer/plastic substrates, characterized in that it comprises the following steps : (a) selection of a polymer/plastic material comprising two or more components which differ with respect to their etching behavior and (b) exposure of said polymer/plastic layer to an etching plasma under conditions appropriate to provide selective removal of one polymer component versus a second plasma-resistant component.
3. A method according to claim 2, whereas the polymer/plastic material is selected among organic-inorganic polymers.
4. A method according to claim 3, whereas the polymer/plastic material is a Siloxane (PDMS).
5. A method according to claim 2, whereas the polymer/plastic material is selected among organic polymers which comprise in their volume and/or on their surface, or on their surface alone, components with different plasma etching behavior, and/or additives and/or impurities.
6. A method according to claim 5, whereas the polymer/plastic material is a Plexiglass (also known as PMMA, with the chemical poly-methyl-methacrylate).
7. A method according to any one of the claims 1 to 6, further comprising the step of plasma or spin coating deposition of a hydrophobic layer on top of said randomly-rough columnar-like surfaces, so as to render the surface super-hydrophobic.
8. Use of the method of claim 7, for the fabrication of surfaces exhibiting very low friction against droplet motion, and thus the use of such surfaces for the control of the transport of droplets using electric fields.
9. Use of the method of claim 7, for the fabrication of surfaces serving as interior surfaces of micro-channels of microfluidic devices.
10. A method according to any one of the claims 1 to 6, further comprising the step of exposure of said surfaces to an oxidizing plasma so as to render the surfaces highly hydrophilic.
11. Use of the method of claim 10, for the fabrication of surfaces serving as interior surfaces of micro-channels of open tubular liquid chromatography or electrochromatography or capillary electrophoresis microdevices for separation purposes.
12. A method according to claim 7 whereas the optical transparency is preserved and the cut-off wavelength is tuned, by controlling the height and aspect ratio of the randomly rough columnar-like surface.
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US20080296260A1 (en) 2008-12-04
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WO2007031799A1 (en) 2007-03-22
GR20050100473A (en) 2007-04-25

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